There is considerable in vitro evidence in the literature which suggests that macrophages from animals with chronic infections or which have been treated with immunomodulating agents play an important role in prevention of tumor development or in resistance to existing tumors. However, the mechanisms by which activated macrophages recognize and kill target cells are not yet clearly understood. This project is designed to examine the effect of soluble macrophage products on the expression of their cytotoxicity in animal systems. The role of soluble factor(s) in the expression of human monocyte-macrophage-mediated lysis of malignant cells will also be evaluated. We also hope to determine if monocyte-macrophages from cancer-bearing patients contain or release these products. It is hoped that these studies will provide further information on the mechanisms of monocyte-macrophage-mediated cytotoxicity.